Measurement and monitoring of gas fluxes is a common practice in many areas of the Earth. For example, in volcanically active regions, the measurement and monitoring of gas fluxes can be an indicator of changes in volcanic activity and could even be critical in saving human lives in the event that CO2 gases are built up in the soil and have the potential for catastrophic release. In the oil industry, measurements of volatile hydrocarbon emissions can be a non-intrusive and inexpensive way of finding potential hydrocarbon deposits. At contaminated sites, flux measurements can either be used to map out the extent and severity of the contamination or can be implemented to determine natural attenuation rates. Moreover, flux measurements are also used extensively in the budgeting of greenhouse gases, both to determine the rate of natural emission of greenhouse gases from soil and groundwater sources as well as to evaluate flux from aquatic systems.
Most commonly, soil gas fluxes are measured using either Fick's law of diffusion (also known as the concentration gradient method), by the static chamber method, or by eddy covariance. All of these methods, however, have some shortcomings.
The concentration gradient method requires the measurement of two or more concentrations at different depths in the soil profile and often involves disturbing the soil to install monitoring equipment. This affects both the soil structure and the gas transport regime. This method, however, typically underestimates fluxes because of improperly constrained diffusion coefficients or effective soil gas diffusivity values. Concentration profiles of natural or injected 222Radon or other tracers have been used to determine diffusivity in the field, but many researchers defer to empirically-derived approximations that require soil-specific input parameters, such as the Millington model or other recent improved models. Unfortunately, diffusivity models tend to perform better in some soils than in others. Alternatively, there are several approaches that allow for laboratory testing of intact soil cores collected in the field. These have the advantage that soil gas diffusivity can be determined on a relatively small spatial and temporal scales that would otherwise be difficult to measure with either 222Radon concentration profiles, or in highly organic substrates, such as soil litter, which are not clearly dealt with in diffusivity model approximations. There remains, however, the potential for changes to soil physical properties (e.g. soil aggregation, compaction etc) that could have a large influence on resulting values.
Aside from diffusivity, other limitations of the gradient approach include the intensiveness of sampling as several simultaneous concentration measurements are required for one flux calculation, frequent gas well installation challenges, the high degree of lateral variability in gas concentrations owing to subsurface heterogeneity, the need for post-processing of data, and the added error resulting from the multiple necessary steps of gas extraction, sample transport, and laboratory analysis.
The static chamber method measures soil gas flux using an accumulator that, in its simplest form, consists of an inverted can placed on the soil surface. Upon deployment, high concentrations of soil gases drive flux out from the soil by diffusion and into the lower concentration headspace of the chamber. Mass accumulation of gas into the chamber over time is used to calculate fluxes. In the past, simple manual chamber techniques have used one or more samples of the chamber headspace over an accumulation period, usually several minutes or more. Gas analysis is typically conducted in the laboratory by gas chromatography. A linear, exponential, or polynomial model is used to fit the data and to calculate fluxes. Many design improvements have been added to sampling chambers over the 60-year history of the technique. Over the last 15 years, good automated chamber instruments have become available for survey CO2 measurement that allow computer control of the technique and gas analysis within the instrument, delivering instant real-time data to the user. Users typically use these instruments as survey tools to sample emissions at various points across the landscape. Permanent chambers are also available with an open top, but with an arm that seals the chamber at a predetermined time interval. Similar to automated survey chambers, these incorporate a detector and microprocessor that can deliver real time data.
Despite the advantages of chamber techniques (simplicity, commercial availability, real-time data), they have several important limitations. Chambers are not suitable for use during winter where snow is present, particularly the permanent deployment types with moving external parts. Commercially available systems are expensive and have been found to underestimate fluxes in certain cases and overestimate them in others. The use of static chambers also requires that a collar be installed on the ground surface, which may temporarily increase gas fluxes and also, if left in the soil for long periods, may aid in the development of a microenvironment that is not indicative of the flux conditions that would be seen elsewhere in the sampling site. There also exist design problems with static chambers in that the estimated flux can be affected by changes in atmospheric pressure as well as by buildup of fluxing gas in the chambers, causing fluxes to slow with time. Other drawbacks include the inability to estimate fluxes from large portions of the soil surface and, in the case of the static chamber methods, the need to return and manually sample the gases built up in the chambers. This latter problem makes a time intensive process that has the possibility for contamination and error. Chambers also provide a non-continuous estimate of gas fluxes since the chamber needs to be manually removed from the soil surface and replaced to initiate a new measurement.
Eddy covariance (also known as micrometeorological technique, eddy correlation or eddy flux) is a method used to estimate gas fluxes over large areas. The basis of this method is the measurement of gas transport vectors from a tower, particularly the vertical turbulent fluxes within atmospheric boundary layers. Simultaneous measurement of vertical transport and concentrations at two or more heights above the ground surface are required to execute the method. This is a technique reserved for highly skilled specialists because it is mathematically complex, requires care in setting up stations, and requires significant data conditioning and post-processing. To date, there is neither a uniform terminology nor a single methodology for the eddy covariance technique.
While the technique can return data on average gas fluxes from a large area (up to several square kilometers), the key assumptions that underlie the method expose obvious limitations. Firstly, fluxes must be fully turbulent, and most of the net vertical transfer must be done by eddies, which means that data cannot be acquired under windless conditions in which eddies are not present, such as frequently occurs at night. The measurement footprint of the station changes constantly according to both wind speed and direction. This means that areas outside the area of interest might be frequently unintentionally monitored. In addition, terrain must be flat and uniform and atmospheric density fluctuations must be negligible. Moreover, eddy covariance stations are extremely expensive to buy, deploy, and run, yet they still suffer from large temporal gaps in collected data. Recent research has shown that the footprint is also a considerable drawback for industrial purposes, such as carbon capture and storage, because eddy covariance stations are not capable of resolving large magnitude emissions from limited areas.
Hence, all of these known methods for measuring the flux of a soil gas still have something left to be desired. There is therefore a clear need for a method of measuring the flux of a soil gases that overcomes the shortcomings of known methods.